Field of the Invention
This invention relates to a method and apparatus for producing three-dimensional objects layer by layer using a powdery material which can be solidified by irradiating it with a beam of charged particles.
Description of Related Art
Equipment for producing a three-dimensional object layer by layer using a powdery material which can be solidified by irradiating it with electromagnetic radiation or an electron beam are known from e.g. U.S. Pat. No. 4,863,538, U.S. Pat. No. 5,647,931 and WO 2004/056511. Such equipment include for instance a supply of powder, means for applying a layer of powder on a work table, and means for directing the beam over the work table. The powder sinters or melts and solidifies as the beam moves or sweeps over a working area of the work table.
When melting or sintering a powder using a high-energy beam, it is important to avoid exceeding the vaporization temperature of the powder, since otherwise the powder will vaporize instead of forming the intended product. US 2005/0186538 discloses a method focusing on this problem. In this method a laser beam repeatedly is directed to the same powder target area during the melting/sintering phase as to step-wise raise the powder temperature. This way, a too high powder temperature is avoided.
When using an electron beam, or other charged particle beam, instead of a laser beam the situation is in some ways different. As the beam of charged particles interacts with the powdery material in a small area it causes heating of the irradiated area but also transfer of charges from the beam particles into the exposed area. The charges are transported out from the exposed area partly through the surrounding powdery material at a rate determined by the electrical conductivity of the powdery material and partly through secondary electron emission from the exposed area. Depending on the intensity of the charged particle beam, the electrical conductivity of the powdery material and the efficiency of secondary electron emission, the powdery material may, in some regions, reach an amount of charging high enough for the electrostatic force between the charged particles at the surface and underlying charged particles to overcome the force of gravity. Thus at this point charged powder particles will lift from and leave the working area and move around above the powder bed. This phenomenon propagates to surrounding areas of the powder layer resulting in a massive powder discharge or lift-off where large amounts of powder will whirl around above the powder bed and perhaps around the entire irradiation chamber. A result of a massive powder lift-off is that the powder particles that have left the working area interfere with the beam and that the structure of the powder layer is destroyed, When this happens, the production process must be interrupted in order to remove the spread-out powder and apply a new layer of powder onto the working table. These procedures take some time to complete and, in the interest of process stability and production rate, it is important to avoid such powder discharges.
Applying the method according to US 2005/0186538 to a powder melting/sintering device equipped with an electron beam is likely to give a poor result since no measures are taken in that method to avoid powder lift-off.
One solution to the problem of avoiding discharges is to add conductive material, such as carbon, to the powder as to increase the electrical conductivity of the powder. Disadvantages of this solution are, however, that the solidifying process of such a powder mixture may be difficult to control and that the properties of the formed product may he affected in a negative way. For instance, the mechanical strength may be decreased.
To lower the probability for powder lifting to occur it may be possible to increase the conductivity in the surrounding powder by performing a more thorough heating or sintering. However, such a procedure increases the time of building a three-dimensional body and because production time is an important parameter this approach is not fully satisfying,
There is thus still a need for improvements in the field of handling lift-off of powder from the work table.